Eutectic colloidal crystal, eutectic colloidal crystal solidified body, and methods for producing them

ABSTRACT

[Resolution means] The eutectic colloidal crystal of the present invention contains two or more kinds of colloidal crystals composed of substantially monodispersed colloidal particles having different particle sizes. This eutectic colloidal crystal is obtained by providing a colloidal dispersion of two or more kinds of colloidal particles having different particle sizes, and a polymer which will not substantially adsorb to the colloidal particles (the coefficient of variation in particle size of these colloidal particles is less than 20%) dissolved in a dispersion medium (dispersion preparation process), and allowing the colloidal dispersion to stand (eutectoid process).

TECHNICAL FIELD

The present invention relates to an aggregate of two or more kinds ofopal-type colloidal crystals which are composed of monodispersedcolloidal particles and mixed together at fixed positions (hereinafterreferred to as “eutectic colloidal crystal” in this description), asolidified body of the eutectic colloidal crystal, and methods forproducing them.

BACKGROUND ART

Colloid means a system in which disperse phases having a size of aboutseveral nm to several μm (e.g., colloidal particles) are dispersed in adispersion medium. The colloidal particles in the colloid can bearranged regularly to form an orderly structure under specialconditions, which is called colloidal crystals.

Similarly to usual crystals, the colloidal crystals Bragg-diffractelectromagnetic waves according to the lattice spacing. The diffractionwavelength can be set to a visible light range by selecting productionconditions (e.g., particle concentration, particle diameter, andrefraction index of particles or medium). Therefore, applicationdevelopment to an optical element or the like including a photonicmaterial has been actively studied both nationally and internationally.The present mainstream of a producing process of an optical materialincludes a multilayer thin film process and a lithography process. Bothof the techniques can produce colloidal crystals having excellentperiodic accuracy. However, the former provides only a one-dimensionalperiodic structure, and the latter provides only the one-dimensionalperiodic structure or a two-dimensional periodic structure.

There are three kinds of colloidal crystals.

The first type is the colloidal crystal in a hard sphere system on whichonly hard sphere repulsion works between particles. This colloidalcrystallization depends only on entropy, and the particle concentrationis the only one concrete parameter. This is similar to a phenomenon thatmacroscopic spheres are regularly arranged when they are stuffed into alimited space, and the volume fraction of the crystallized particles isabout 0.5 (concentration=50% by volume) or more. At this time,crystallization occurs even if the particles are not in contact witheach other.

The second type is the opal crystal, which is the generic name of acrystal structure packed with particles in contact with each other. Thevolume fraction depends on the crystal structure, and is, for example,about 0.68 for a body-centered cubic lattice, and 0.74 for aface-centered cubic lattice.

The third type is the charged colloidal crystal, which is formed byelectrostatic interaction working between particles in a dispersionsystem of charged colloidal particles (charged colloidal system). Theelectrostatic interaction extends for a long distance, so that crystalscan be formed even when the particle concentration is low (theinterparticle distance is long), and the particle volume fraction isabout 0.001.

There is a report that the colloidal particles having a uniform particlesize precipitated, aggregated, and regularly arranged when they wereused in a colloidal system with no special interaction between colloidalparticles, and form closest-packed opal-type colloidal crystals (PatentLiterature 1). However, for the opal-type colloidal crystals, only anaggregate composed of colloidal crystals having one lattice constant hasbeen obtained, and there is no report on a eutectic colloidal crystalcontaining two or more kinds of colloidal crystals composed ofmonodispersed colloidal particles.

As an example of deposition of colloidal crystals from a multi-componentcolloid, Non-Patent Literature 1 reports the classification of gold fineparticles and gold nano-rods, but the above-described eutectic colloidalcrystal was not obtained therein. More specifically, this documentreports that, from the mixed colloid of gold fine particles and goldnano-rods, only the gold fine particle alone formed colloidal crystalsand gold nano-rods aggregated at the grain boundaries, which does notmean the formation of a eutectic colloidal crystal composed of thecolloidal crystals of gold nano-rods and gold fine particles.

In a recently found phenomenon, plural kinds of opal-type colloidalcrystals having different lattice constants coexisted in a dispersionmedium (Non-Patent Literature 2). However, this colloidal crystal systemis a state where plural kinds of opal-type colloidal crystals aresuspended in a dispersion medium having the same specific gravity, butnot mixed together at fixed positions to form an aggregate. Therefore,the direction of the optical axis of the colloidal crystals can bevaried by Brownian movement, and thus the application to optical devicessuch as photonic materials is difficult.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No.    2004-109178

Non-Patent Literatures

-   Non-Patent Literature 1: Kyoungweon P, et. al. Nanoletters 10,    1433-1439 (2010)-   Non-Patent Literature 2: Anna Kozina, et. al. Soft Matter, 10,    9523-9533 (2014).

SUMMARY OF INVENTION Technical Problems

The present invention has been accomplished in view of theabove-described present circumstances, and is intended to provide a“eutectic colloidal crystal”, which is an aggregate of two or more kindsof colloidal crystals which are composed of substantially monodispersedcolloidal particles and mixed at fixed positions, a solidified body ofthe eutectic colloidal crystal, and methods for producing them.

Solutions to Problems

The inventors carried out the following study on the process offormation of colloidal crystals from colloidal particles dispersed in acolloid in the production of opal-type colloidal crystals.

When colloidal particles and a polymer, which will not adsorb to thecolloidal particles, are dispersed in a dispersion medium, the colloidalparticles approach each other to form narrow regions sandwiched betweenthe colloidal particles. When the narrow regions are lessened to adegree that the polymer dispersed in the dispersion medium cannot enter,a polymer concentration difference arises between the narrow regions andother wider regions. This concentration difference causes an osmoticpressure difference, and thus an attraction works between the colloidalparticles, and colloidal particles aggregate. For the cases ofmonodispersion where all the colloidal particles have a uniform particlesize, closest-packed colloidal crystals having an equal lattice constantare formed. On the other hand, the particles contained in themonodispersed colloidal particles as impurities are ejected between thecolloidal crystals without being taken into the colloidal crystals(e.g., see Non-Patent Literature 1)

The inventors further carried out a detailed study on the process offormation of colloidal crystals in the case where plural kinds ofmonodispersed colloidal particles having different particle sizes aremixed together in a colloid. As a result of this, a surprising eutectoidphenomenon was found; when the specific gravity of a dispersion mediumis smaller than that of colloidal particles, among the plural kinds ofcolloidal particles, one kind of the colloidal particles becomescolloidal crystals, while the other kinds of the colloidal particles areejected out to the region out of the colloidal crystals, and precipitatein the dispersion medium while forming other colloidal crystals havingdifferent lattice constants in the ejected region, and finally become anaggregate at the position where the plural colloidal crystals orientedon the bottom of the container are fixed (i.e., eutectic colloidalcrystal), and thus the present invention has been accomplished.

More specifically, the eutectic colloidal crystal of the presentinvention contains two or more kinds of opal-type colloidal crystalscomposed of colloidal particles having a coefficient of particle sizevariation of less than 20%, the colloidal crystals being mixed togetherat fixed positions.

The eutectic colloidal crystal of the present invention contains two ormore kinds of opal-type colloidal crystals composed of colloidalparticles having a coefficient of particle size variation of less than20%, the colloidal crystals being mixed together at fixed positions. Inother words, opal-type colloidal crystals having different latticeconstants are intermingled. Therefore, the diffraction color can becontrolled by, for example, adjusting the combination and abundanceratio of these colloidal crystals. In addition, the eutectic colloidalcrystal resists color fading because it is free of dyes, and thus ispromising as a novel coloring material.

In addition, since the positions of the two or more kinds of opal-typecolloidal crystals are fixed, the light axial direction of the pluralcolloidal crystals will not vary, different from the system described inNon-Patent Literature 2 where plural colloidal crystals are suspended ina dispersion medium. Therefore, the eutectic colloidal crystal issuitable for the use in optical devices such as photonic materials.

The coefficient of variation (CV) of the particle size means the valueof (standard deviation of particle size×100/average particle size), andis preferably less than 15%, more preferably less than 12%, even morepreferably less than 11%, yet more preferably less than 10%, and mostpreferably about 8% or less.

The eutectic colloidal crystal of the present invention can be producedas follows.

The method for producing the eutectic colloidal crystal of the presentinvention includes: a dispersion preparation process of preparing acolloidal dispersion where two or more kinds of colloidal particleshaving different particle sizes are dispersed in a dispersion mediumwhich dissolves a polymer, the coefficient of variation of the particlesize of the colloidal particles is less than 20%, and the specificgravity of the dispersion medium is smaller than that of the colloidalparticles; and a eutectoid process of depositing two or more kinds ofopal-type colloidal crystals having different lattice constants byallowing the colloidal dispersion to stand.

In this method for producing a eutectic colloidal crystal, firstly, asthe dispersion preparation process, a colloidal dispersion is preparedin which two or more kinds of colloidal particles having differentparticle sizes are dispersed in a dispersion medium which dissolves apolymer, the coefficient of variation of the particle size of thecolloidal particles is less than 20%, and the specific gravity of thedispersion medium is smaller than that of the colloidal particles. Theproportion of the particle size between the colloidal particles havingdifferent particle sizes is preferably more than 1.03, more preferably1.05 or more, and most preferably 1.1 or more. In addition, the totalvolume fraction of the colloidal particles to the dispersion ispreferably from 0.001 to 0.1, and more preferably from 0.002 to 0.05.

Then, as the eutectoid process, the colloidal dispersion is allowed tostand. In this eutectoid process, of the plural kinds of colloidalparticles, when one type of the colloidal particles becomes opal-typecolloidal crystals, the other colloidal particles are ejected to theregion out of the opal-type colloidal crystals, and form other opal-typecolloidal crystals having a different lattice constant in the ejectedregion. The specific gravity of the dispersion medium is smaller thanthat of the colloidal particles, so that the plural opal-type colloidalcrystals sediment in the dispersion medium while being formed, andfinally become an aggregate of plural colloidal crystals (morespecifically eutectic colloidal crystal), the lattice planes of thecolloidal crystals being oriented on the bottom of the container.

The colloidal particles used for producing the eutectic colloidalcrystal are not particularly limited, and may be organic or inorganicparticles. The organic particles may be the particles of a polymer suchas polystyrene or an acrylic resin (e.g., polyacrylate,polymethacrylate, or polyacrylonitrile). The inventors used theparticles of polystyrene as the colloidal particles, and confirmed thateutectic colloidal crystal was certainly obtained. In addition, examplesof the inorganic particles include silica, alumina, titanium oxide,gold, and silver.

According to the test results provided by the inventors, the particlesize of the colloidal particles is preferably from 1 nm to 50 μm. If theparticle size is less than 1 nm, sedimentation of the colloidalparticles in the eutectoid process is too slow, and the productionrequires a long time. On the other hand, if the colloidal particles arelarger than 50 μm, sedimentation is so fast that the crystals in theeutectic colloidal crystal tends to be disturbed. The particle size isparticularly preferably from 10 nm to 1 μm. In order to adjust thesedimentation rate, the specific gravity of the dispersion medium ispreferably selected as appropriate. The specific gravity of thedispersion medium can be adjusted by mixing with deuterium oxide or anorganic solvent, and dissolving a low molecular solute therein.

The colloidal crystals of the present invention may be a eutecticcolloidal crystal solidified body which has been solidified by animmobilizing agent. The eutectic colloidal crystal solidified body has adramatically improved mechanical strength, and offers markedly easyhandling.

A eutectic colloidal crystal solidified body is easily produced by usingan immobilizing agent such as a photocurable resin. More specifically,the method of the present invention for producing a eutectic colloidalcrystal solidified body includes: a dispersion preparation process ofpreparing a colloidal dispersion where two or more kinds of colloidalparticles having different particle sizes are dispersed in a dispersionmedium which dissolves a polymer and a photocurable resin, and thespecific gravity of the dispersion medium is smaller than that of thecolloidal particles; a eutectoid process of depositing two or more kindsof opal-type colloidal crystals having different lattice constants byallowing the colloidal dispersion to stand; and a photoirradiationprocess of immobilizing the eutectic colloidal crystal formed in theeutectoid process by photoirradiation.

Examples of the immobilizing agent include a solution containing a gelmonomer, a crosslinking agent, and a photopolymerization initiator.Examples of the gel monomer include vinyl monomers such as acrylamideand derivatives thereof, examples of the crosslinking agent includeN,N′-methylenebisacrylamide, and examples of the photopolymerizationinitiator include 2,2′-azobis[2-methyl-N-[2-hydroxyethyl]-propionamide]. Other examplesinclude a water soluble photosensitive resin composed of a polyvinylalcohol with pendant azide photosensitive group. Alternatively, eutecticcolloidal crystal are formed in a resin monomer, followed bysolidification.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a eutectic colloidal crystal of anembodiment.

FIG. 2 is a schematic view showing a production process of the eutecticcolloidal crystal of the embodiment.

FIG. 3 is a schematic view showing a depletion effect in the case wherecolloidal particles are dispersed in a polymer solution.

FIG. 4 is a schematic view showing a formation process of the eutecticcolloidal crystal.

FIG. 5 is an optical micrograph of the eutectic colloidal crystal ofExample 1 (upper left: the case using a filter transmitting red color,lower left: the case using a filter transmitting green color, right: aphotograph overlaying the upper left and lower left photographs).

FIG. 6 is an optical micrograph of the eutectic colloidal crystal ofExample 1.

FIG. 7 shows Fourier transform image photographs of the crystal grainsof the eutectic colloidal crystal of Example 1.

FIG. 8 shows an optical micrograph of the eutectic colloidal crystal ofExample 2 (left), a schematic view showing diffraction in the particles(center), and a reflection spectrum (right).

FIG. 9 shows an optical micrograph of the eutectic colloidal crystal ofExample 3 (left), a schematic view showing diffraction in the particles(middle), and a reflection spectrum (right).

FIG. 10 is an optical micrograph of the eutectic colloidal crystal fixedby the photocurable resin of Example 4.

FIG. 11 shows reflection spectra of the eutectic colloidal crystal inExamples 5 to 8.

FIG. 12 is a phase diagram of the eutectic colloidal crystal when fourkinds of polystyrene colloidal particles (PS-1 to PS4) were used (theordinate is the particle size ratio, and the abscissa is volume fraction(expressed in %)).

FIG. 13 shows reflection spectra of the eutectic colloidal crystal inExamples 9 to 11.

FIG. 14 is a phase diagram of the eutectic colloidal crystal when fourkinds of silica colloidal particles (PS-1 to PS4) were used (theordinate is the particle size ratio, and the abscissa is volume fraction(expressed in %)).

FIG. 15 shows reflection spectra in Example 12 and Comparative Examples1 and 2.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an example of the eutectic colloidal crystal of the presentinvention. The eutectic colloidal crystal is composed of three kinds ofcolloidal particles having different particle sizes d1, d2, and d3,wherein the colloidal crystals 1, colloidal crystals 2, and colloidalcrystals 3, each composed of a single kind of colloidal particles, areintermingled. These colloidal crystals take a closest-packed structure.The lattice planes of the colloidal crystals are oriented so as to be inparallel to the paper plane.

The eutectic colloidal crystal can be produced according to the processshown in FIG. 2.

(Dispersion Preparation Process S1)

Firstly, as the dispersion preparation process S1, a solution of apolymer dissolved in a dispersion medium is prepared. Two or more kindsof monodispersed colloidal particles (three kinds in FIG. 2, but may betwo, four, or more) having different particle sizes are added to thesolution, and they are stirred to make a colloidal dispersion.Alternatively, each kind of the colloidal particles may be dispersed inadvance in separate solvents to make dispersions. The dispersion mediumis selected from those having a specific gravity smaller than that ofthe colloidal particles. In the preparation of the colloid, dispersionmay be accelerated using an ultrasonic generator, which may be togetherwith any other device. The dispersion medium is not particularly limitedas long as it dissolves a polymer, and may be water or other organicsolvent such as alcohol. The polymer is not particularly limited as longas it forms colloidal crystals, and may be an ionic polymer or anonionic polymer.

Examples of the water-soluble nonionic polymer include polyethyleneglycol, polypropylene glycol, polyvinyl alcohol, polyether, andpolyvinyl pyrrolidone. Examples of the water-soluble ionic polymerinclude cationic polymers such as polyvinylpyridine, polyvinyl benzylammonium, and polypeptide; and anionic polymers such as polyacrylicacid, polyacrylamide, polypeptide, and natural polymers such aspolysaccharides. In addition, examples of the hydrophobic polymerinclude polystyrene and polydimethylsiloxane (the solvent for them maybe toluene or xylene).

In the selection of these polymers, the polymers having a chargeopposite to that of the surface charge of the colloidal particles arenot preferred, because they adsorb to colloidal particles to form acharged colloid. However, even the polymer having a charge opposite tothat of the surface charge of the colloidal particles can be used byincreasing the salt concentration, which markedly thins the electricdouble layer, allows the approach of the colloidal particles to eachother, forms a hard sphere colloid, and finally, as will be describedlater, the difference in the polymer concentration causes the differencein the osmotic pressure to generate depletion attraction, and formsopal-type colloidal crystals. Furthermore, this phenomenon can bepositively used to adjust the salt concentration in the colloid asappropriate to control the thickness of the electric double layer, whichallows the control of, for example, the attraction between the colloidalparticles, the growth rate of the opal-type colloidal crystals, and eventhe half width to the light of the colloidal crystals. Examples of themethod for controlling the salt concentration include the control of theabundance of ionic groups (carboxylates, amino groups, etc.) inpolymers, and addition of salts.

Alternatively, the eutectic colloidal crystal may be produced by adding,in addition to the colloidal particles forming opal-type colloidalcrystals, the particles finer than the colloidal particles as a polymer,and making the added particles exert the below-described depletionattraction.

(Eutectoid Process S2)

In the next place, colloidal dispersion prepared in the dispersionpreparation process S1 is taken by a dropper or the like, placed on aglass bottom dish or the like, and allowed to stand (eutectoid processS2). In the eutectoid process S2, the colloidal particles having thesame particle size attract each other to flocculate, and a eutecticcolloidal crystal composed of three or more kinds of colloidal crystalsC1, C2, and C3 having different lattice constants is formed.

The reason for the attraction of the colloidal particles is presumed asfollows. More specifically, the polymer added to the colloidaldispersion is dissolved in the dispersion medium, and intrinsicallydispersed in the dispersion medium uniformly. However, as shown in FIG.3, narrow regions which are too small for the entry of the polymer areformed when the colloidal particles have approached each other.Therefore, the narrow regions become depletion regions where no polymeris present, and a polymer concentration difference arises between thenarrow regions and other bulk regions. Because of the osmotic pressuredifference, the colloidal particles approach and contact with eachother, and form colloidal crystals. Accordingly, when the added polymeradsorbs to the colloidal particles, such depletion regions are unlikelyto be formed, and the osmotic pressure difference is unlikely to occur,which is not preferred for the production of colloidal crystals.

In the formation process of the eutectic structure of the colloidalparticles, a behavior similar to eutectic formation in the atomic andmolecular system is observed. More specifically, the approach betweenthe colloidal particles is made between the particles having the sameparticle size to form the first colloidal crystals C1, and the colloidalparticles having the other particle size gather at the grain boundariesof the first colloidal crystals (left of FIG. 4). In this manner, amongthe colloidal particles gathered at the grain boundaries of the firstcolloidal crystals C1, the colloidal particles having the same particlesize further gather to form the second colloidal crystals C2 (middle ofFIG. 4). Furthermore, the colloidal particles gathered at the grainboundaries of the first and second colloidal crystals C1 and C2 form thethird colloidal crystals C3 (right of FIG. 4).

During the formation of the colloidal crystals C1, C2, and C3, thecolloidal crystals C1, C2, and C3 composed of the colloidal particleshaving a higher specific gravity than the dispersion medium keep onsedimentation. And finally they precipitate on the bottom of thecontainer, and form an aggregate (more specifically, eutectic colloidalcrystal) at the positions where the colloidal crystals C1, C2, and C3are fixed. At this time, the colloidal crystals are oriented on thebottom.

Examples further embodying the present invention are described below.

EXAMPLE 1

In Example 1, a eutectic colloidal crystal was made using three kinds ofmonodispersed polystyrene particles. More specifically, the firstpolystyrene particles were spherical charged polystyrene particles PS600(Thermo Scientific, purchased in the form of an aqueous dispersion withdiameter d=600 nm and volume fraction=0.05, coefficient of variation inparticle size: 3%), the second polystyrene particles were sphericalgreen fluorescent charged polystyrene particles G500 (commercialproduct, diameter d=500 nm, volume fraction=0.1, coefficient ofvariation in particle size: 5%), and the third polystyrene particleswere spherical red fluorescent charged polystyrene particles DR390(commercial product, diameter d=about 390 nm, volume fraction=0.01,coefficient of variation in particle size: 5%). These particles werepurified by a dialysis method and an ion exchange method, mixed at theratio of the first polystyrene particles:second polystyreneparticles:third polystyrene particles=1:0.1:0.05 (volume ratio) (200 μLin total), further 200 μL of a 0.1 w % sodium polyacrylate PAANa(molecular weight: one million, degree of neutralization: 50%) aqueoussolution was added, and stirred to make a hard sphere colloid sample.400 μL of the hard sphere colloid sample was placed in a glass bottomdish container, allowed to stand for several hours to one week, andobserved with an inverted optical microscope. The results are shown inFIGS. 5 and 6. The upper left of FIG. 5 is the photograph of the caseusing a filter transmitting red color, the lower left is the photographof the case using a filter transmitting green color, and the right isthe photograph showing the case using no filter. These photographs showthat the first to third polystyrene particles aggregated separately. Inaddition, FIG. 6 shows that the aggregated first to third polystyreneparticles took on a closest-packed structure to form a eutecticcolloidal crystal, which is an aggregate of colloidal crystals.

Furthermore, as shown in FIG. 7, the Fourier transform images of thecrystal grains show that the colloidal particles grow in the crystalstructures to form a mixture of three kinds of colloidal crystals (morespecifically eutectic colloidal crystal), and all of these colloidalcrystals are oriented on the bottom of the container.

EXAMPLE 2

In Example 2, a eutectic colloidal crystal was made using two kinds ofmonodispersed polystyrene particles. More specifically, the firstpolystyrene particles were spherical charged polystyrene particles PS200(Thermo Scientific, diameter d=200 nm, volume fraction=0.1, coefficientof variation in particle size: 5%), and the second polystyrene particleswere spherical charged polystyrene particles PS250 (synthesized by theinventors, diameter d=250 nm, volume fraction=0.068, coefficient ofvariation in particle size: 15%).

The polystyrene particles were synthesized by a soap free emulsionpolymerization method. More specifically, 210 mL of water, 100 mL ofmethanol, 20 mL of styrene monomer, 0.15 g of sodium p-styrenesulfonateas an anionic comonomer, and 1 mL of divinylbenzene were mixed, andstirred for about 30 minutes in a constant temperature bath at 80° C.,at a rotation speed of 300 rpm, and in an argon atmosphere. Thereafter,0.1 g of potassium peroxodisulfate as a radical polymerization initiatorwas added and stirred for 7 hours, thus synthesizing polystyreneparticles.

These particles were purified by a dialysis method and an ion exchangemethod, mixed at a ratio of the first polystyrene particles:secondpolystyrene particles=25:1 (volume ratio) (75 μL in total), further 200μL of sodium polyacrylate PAANa (molecular weight:million, degree ofneutralization:50%, 0.1 w % aqueous solution) was added, and stirred tomake a colloid sample. The procedure thereafter is the same as that inExample 1, so that the explanation thereof is omitted.

The precipitate formed on the glass bottom dish as described above weresubjected to optical microphotographing and reflection spectrummeasurement (fiber spectrometer, Ocean Optics, USB2000). The results areshown in FIG. 8. The microphotograph shown at the left of FIG. 8indicates that, of the two kinds of particles, the particles having thesame particle size were gathered. In addition, in the reflectionspectrum on the right side, a reflection spectrum at 510 nm generated byclosest-packing of 200 nm particles, and a reflection spectrum at 630 nmgenerated by closest-packing of 250 nm particles were observed, whichindicates that closest-packed colloidal crystals had been formed.

EXAMPLE 3

In Example 3, a eutectic colloidal crystal was made using three kinds ofmonodispersed polystyrene particles. More specifically, the firstpolystyrene particles were spherical charged polystyrene particles PS200(Thermo Scientific, diameter d=200 nm, volume fraction=0.1), the secondpolystyrene particles were spherical charged polystyrene particles PS250(synthesized by the inventors (the particles used in Example 2),diameter d=250 nm, volume fraction=0.068), and the third polystyreneparticles were spherical charged polystyrene particles PS300 (ThermoScientific, diameter d=about 300 nm, volume fraction=0.1, coefficient ofvariation in particle size: 3%). These particles were purified by adialysis method and an ion exchange method, mixed at the ratio of thefirst polystyrene particles:second polystyrene particles:thirdpolystyrene particles=4:3:5 (volume ratio) (100 μL in total), further200 μL of a 0.1 w % sodium polyacrylate PAANa (molecular weight:onemillion, degree of neutralization 50%) aqueous solution was added, andstirred to make a colloidal dispersion. The procedure thereafter is thesame as that in Example 1, so that the explanation thereof is omitted.

The precipitate formed on the glass bottom dish as described above weresubjected to optical microphotographing and reflection spectrummeasurement (fiber spectrometer, Ocean Optics, USB2000). The results areshown in FIG. 9. The microphotograph shown at the left of FIG. 9indicates that, of the three kinds of particles, the particles havingthe same particle size were gathered together. In addition, in thereflection spectrum on the right side, a reflection spectrum at 510 nmgenerated by closest-packing of 200 nm particles, a reflection spectrumat 630 nm generated by closest-packing of 250 nm particles, and areflection spectrum at 790 nm generated by closest-packing of 300 nmparticles were observed, and further a reflection spectrum appeared at410 nm, which is presumed to be a secondary diffraction line of thecolloidal crystals of 300 nm particles.

EXAMPLE 4

<Preparation of Eutectic Colloidal Crystal Solidified Body>

In Example 4, the eutectic colloidal crystal was fixed by photocurablehydrogel using two kinds of monodispersed polystyrene particles.

More specifically, spherical charge polystyrene particle PS600 (ThermoScientific, diameter d=200 nm, volume fraction=0.1) as the firstpolystyrene particles, and spherical charge polystyrene particle PS430(synthesized by the inventors (the particles used in Example 2, diameterd=430 nm, volume fraction=0.068) as the second polystyrene particleswere purified by a dialysis method and an ion exchange method, mixed atthe ratio of the first polystyrene particles:second polystyreneparticles=5:1 (volume ratio) (50 μL in total), further 200 μL of a 0.1wt % aqueous solution of sodium polyacrylate PAANa (molecular weight:onemillion, degree of neutralization 50%), 250 μL of the following gellingagent, and 500 μL of water were added and stirred to make a colloidaldispersion. The procedure thereafter is the same as that in Example 1;after confirming the formation of the eutectic colloidal crystal by anoptical microscope, polymerization of the gelling agent was initiated byultraviolet irradiation, whereby a eutectic colloidal crystal solidifiedbody was obtained.

Composition of Gelling Agent

Gel monomer: N,N′-dimethylol acrylamide (N-MAM) 0.67 mol/L

Crosslinking agent: methylenebisacrylamide (BIS) 10 mmol/L

Photopolymerization initiator:

-   2,2′-azobis[2-methyl-N-[2-hydroxyethyl]-propionamide 4 mg/mL

As a result of this, as shown in FIG. 10, it was found that two kinds ofcolloidal crystals having different lattice constants were maintained ina state firmly fixed by the gelling agent.

<Influence of Particle Size on Formation of Eutectic Colloidal Crystal>

In order to determine how much difference in colloidal particle size isnecessary for the formation of a eutectic colloidal crystal in theproduction of a eutectic colloidal crystal composed of different twokinds of monodispersed colloidal particles, the following experiment wascarried out using the colloidal particles of various particle sizes.

More specifically, the polystyrene (PS) particles (Thermo Scientific andothers, PS-1 to PS-5) and silica particles (Nippon Shokubai Co., Ltd,S-1 to S-4) having various particle sizes shown in Table 1 were used,and sodium polyacrylate (NaPAA), which had been prepared by adding NaOHto polyacrylic acid (Wako Pure Chemical Industries, Ltd.) to make asample with a degree of neutralization of 50%, was used as a polymer. Inaddition, two kinds of colloidal particles dispersions and 0.1 wt %NaPAA were mixed so as to make the total concentration of the two kindsof colloidal particles 3.0 vol %, thus producing a eutectic colloidalcrystal. Table 1 shows the particle size d (nm) and the number ofsurface charges Z (count/particle) of the colloidal particles. Table 2shows the combination of the two-component colloidal systems and theratio of the particle size of the two kinds of colloidal particles usedin the experiment.

TABLE 1 Polystyrene PS-1 PS-2 PS-3 PS-4 PS-5 d (nm) 171 205 211 247 282Z (/particles) 561 649 767 745 1302 Silica S-1 S-2 S-3 S-4 d (nm) 189204 264 271 Z (/particles) 719 608 1422 1361

TABLE 2 Particle size Large Small of (L)/particle particles (L)particles (S) size of (S) Example 5 PS-5 PS-1 1.65 Example 6 PS-4 PS-11.44 Example 7 PS-3 PS-1 1.23 Example 8 PS-3 PS-2 1.03 Example 9 S-3 S-21.29 Example 10 S-2 S-l 1.08 Example 11 S-4 S-3 1.03

The Case Using Polystyrene Particles as Colloidal Particles

The reflection spectrum was measured two to four days after thepreparation of the eutectic colloidal crystal. As a result of this, inExamples 5 to 8 where polystyrene particles were used as colloidalparticles, as shown in FIG. 11, except for Example 8 where the particlesize ratio was as small as 1.03, two diffraction peaks were clearlyobserved, which indicates the formation of a eutectic colloidal crystal.On the other hand, in Example 8, a single diffraction peak was observed.The reason for this is likely that a eutectic colloidal crystal havingadjacent diffraction wavelengths was formed, or a solid solution wasformed because the particle size ratio was as small as 1.03.

(Drawing of Phase Diagram)

Using the four kinds of polystyrene colloidal particles (PS-1 to PS-4)shown in Table 1, colloidal dispersions were prepared at variousparticle size ratios (r_(L)/r_(S)) and various volume fractions, andallowed to stand. Thereafter, the reflection spectra of the precipitateswere measured, and the presence or absence of the generation ofcolloidal crystals was examined, thereby drawing a phase diagram. Theresult is shown in FIG. 12. The marks ◯ show the conditions under whichtwo diffraction peaks were observed in the reflection spectrummeasurement (more specifically, the conditions under which a eutecticcolloidal crystal was observed), and the marks Δ show the conditionsunder which a single diffraction peak was observed in the reflectionspectrum measurement (more specifically, the conditions under which noeutectic colloidal crystal was observed).

The Case Using Silica Particles as Colloidal Particles

The reflection spectrum was measured two to four days after thepreparation of the eutectic colloidal crystal. As a result of this, inExamples 9 to 11 where silica particles were used as colloidalparticles, as shown in FIG. 13, except for Example 11 where the particlesize ratio was as small as 1.03, two diffraction peaks were clearlyobserved, which indicates that a eutectic colloidal crystal was formed.On the other hand, in Example 11, a single diffraction peak wasobserved. The reason for this is likely that a eutectic colloidalcrystal having adjacent diffraction wavelengths was formed, or a solidsolution was formed because the particle size ratio was as small as1.03.

(Drawing of Phase Diagram)

Using the four kinds of silica colloidal particles (S-1 to S-4) shown inTable 1, colloidal dispersions were prepared at various particle sizeratios (r_(L)/r_(S)) and various volume fractions, and allowed to stand.Thereafter, the reflection spectra of the precipitates were measured,and the presence or absence of the generation of colloidal crystals wasexamined, thereby drawing a phase diagram. The result is shown in FIG.14. The meanings of the marks ◯ and Δ are the same as the case usingpolystyrene colloidal particles.

<Formation of Eutectic Colloidal Crystal Composed of PolystyreneParticles and Silica Particles>

In Example 12, polystyrene colloidal particles PS-5 (2.5 vol %) andsilica colloidal particles S-2 (:0.5 vol %) were mixed to make acolloidal dispersion, and this dispersion was allowed to stand toprepare a eutectic colloidal crystal composed of polystyrene colloidalcrystals and silica colloidal crystals.

On the other hand, in Comparative Example 1, a colloidal dispersion wasprepared from polystyrene colloidal particles PS-5 (3 vol %) alone, andthe dispersion was allowed to stand to prepare polystyrene colloidalcrystals.

In Comparative Example 2, a colloidal dispersion composed of silicacolloidal particles S-2 (3.0 vol %) alone was prepared, and allowed tostand to prepare silica colloidal crystal.

The eutectic colloidal crystal of Example 12 and the colloidal crystalsof Comparative Examples 1 and 2 thus prepared were measured for thereflection spectra 2 to 4 days after preparation.

As a result of this, as shown in FIG. 15, in Example 12 where thecrystals were prepared from the polystyrene colloidal particles PS-5 andsilica colloidal particles S-2, the primary and secondary peaks of thepolystyrene colloidal crystals, and the primary peak of the silicacolloidal crystals were observed. On the other hand, the primary andsecondary peaks of the polystyrene colloidal crystals were observed inComparative Example 1, and the primary peak of the silica colloidalcrystals was observed in Comparative Example 2.

According to these results, it was found that a eutectic colloidalcrystal was obtained even by mixing colloidal particles of differentcomponents, or silica colloidal particles and polystyrene colloidalparticles.

The present invention is not limited to the description of the examplesof the invention in any way. The present invention includes variousmodification aspects capable of being easily conceived by a personskilled in the art without departing from the description of claims.

INDUSTRIAL APPLICABILITY

The eutectic colloidal crystal of the present invention is useful as amodel system of atomic and molecular crystals, and as a technical toolof research and development in the field of crystallography. Inparticular, since the particles of colloidal crystals can be observed byoptical microscopes, they are easier to be observed than atomic andmolecular crystals. In addition, since many kinds of colloidal crystalsare intermingled, for example, the adjustment of the abundance ratiobetween them allows the control of the diffraction color. In addition,it is resistant to color fading, and thus is promising as a novelcoloring material which develops mixed colors of three primary colors.In addition, it can be used as an electron material for optical filters,or as a decoration material for smartphone covers and nail materials.

REFERENCE SIGNS LIST

1, 2, 3 colloidal crystals (eutectic colloidal crystal)

The invention claimed is:
 1. A eutectic colloidal crystal comprising twoor more kinds of opal-type colloidal crystals, each said two or morekinds of opal-type colloidal crystals being composed of different typesof colloidal particles, each type of the colloidal particles having adifferent particle size and having a coefficient of particle sizevariation of less than 20% the colloidal particles being mixed togetherat fixed positions in the eutectic colloidal crystal, wherein thecoefficient of particle size variation is a value of a standarddeviation of particle diameter×100/an average particle diameter.
 2. Theeutectic colloidal crystal of claim 1, wherein the lattice planes of thetwo or more kinds of opal-type colloidal crystals are oriented on thesame plane.
 3. The eutectic colloidal crystal of claim 1, wherein thetwo or more kinds of opal-type colloidal crystals are composed ofpolymer colloidal particles.
 4. The eutectic colloidal crystal of claim3, wherein the polymer colloidal particles are composed of polystyreneparticles.
 5. The eutectic colloidal crystal of claim 1, wherein theaverage particle size of each type of the colloidal particles is from 1nm to 50 μm.
 6. The eutectic colloidal crystal of claim 1, wherein thetwo or more kinds of opal-type colloidal crystals are composed of silicaparticles.
 7. A eutectic colloidal crystal solidified body prepared byimmobilizing the eutectic colloidal crystal of claim 1 by animmobilizing agent.
 8. The eutectic colloidal crystal of claim 1,wherein said two or more kinds of opal-type colloidal crystals havedifferent lattice constants from each other.
 9. The eutectic colloidalcrystal of claim 1, wherein said two or more kinds of opal-typecolloidal crystals are composed of polystyrene particles, and said twoor more kinds of opal-type colloidal crystals have different latticeconstants from each other.
 10. A method for producing eutectic colloidalcrystal comprising: a dispersion preparation process of preparing acolloidal dispersion where two or more kinds of opal-type colloidalcrystals, each said two or more kinds of opal-type colloidal crystalsbeing composed of different types of colloidal particles, each type ofthe colloidal particles having a different particle size are dispersedin a dispersion medium which dissolves a polymer, the coefficient ofvariation of the particle size of the colloidal particles is less than20% the colloidal particles being mixed together at fixed positions inthe eutectic colloidal crystal, and the specific gravity of thedispersion medium is smaller than that of the colloidal particles; and aeutectoid process of depositing the two or more kinds of opal-typecolloidal crystals having different lattice constants by allowing thecolloidal dispersion to stand.
 11. The method for producing the eutecticcolloidal crystal of claim 10, wherein the two or more kinds ofopal-type colloidal crystals are composed of polymer colloidalparticles.
 12. The method for producing the eutectic colloidal crystalof claim 10, wherein the polymer colloidal particles are composed ofpolystyrene particles.
 13. The method for producing the eutecticcolloidal crystal of claim 10, wherein the two or more kinds ofopal-type colloidal crystals are composed of silica particles.
 14. Themethod for producing the eutectic colloidal crystal of claim 10, whereinthe average particle size of each type of the colloidal particles isfrom 1 nm to 50 μm.
 15. The method for producing the eutectic colloidalcrystal of claim 10, wherein the volume fraction of the colloidalparticles to the colloidal dispersion is from 0.001 to 0.1.
 16. A methodfor producing a eutectic colloidal crystal solidified body comprising: adispersion preparation process of preparing a colloidal dispersion wheretwo or more kinds of opal-type colloidal crystals, each said two or morekinds of opal-type colloidal crystals being composed of different typesof colloidal particles, each type of the colloidal particles having adifferent particle size are dispersed in a dispersion medium whichdissolves a polymer and a photocurable resin, the colloidal particlesbeing mixed together at fixed positions in the eutectic colloidalcrystal, the coefficient of variation of the particle size of thecolloidal particles is less than 20% and the specific gravity of thedispersion medium is smaller than that of the colloidal particles; aeutectoid process of depositing the two or more kinds of opal-typecolloidal crystals having different lattice constants by allowing thecolloidal dispersion to stand; and a photoirradiation process ofimmobilizing the eutectic colloidal crystal formed in the eutectoidprocess by photoirradiation.